(a) Field of the Invention
The present invention relates to an exhaust manifold of an internal combustion engine, and, more particularly, to an improved exhaust manifold capable of reducing exhaust emissions by pre-activating catalysts with a high exhaust temperature, the temperature being maintained by transforming profiles of runners and a connecting angle between the runners and a mixing pipe of the exhaust manifold to minimize heat loss caused by friction of the exhaust gas on a wall of the mixing pipe.
(b) Description of the Related Art
Generally, internal combustion engines generate power to drive vehicles by combusting fuels. Exhaust gases resulting as by-product of this operation are emitted from the engine. Since a relatively excessive amount of exhaust gases is generated during engine warm-up period, the time required for the catalytic converter to activate determines how much exhaust gas is generated during this initial period after the vehicle start-up. The exhaust manifold acts to lead the exhaust gases to the exhaust pipe in which the catalytic converter is installed.
FIG. 1.about.FIG. 3 show a conventional exhaust manifold. As shown in the drawings, first, second, third, and fourth runners 2, 3, 4, and 5 are connected to an engine cylinder 1 at their one ends, and at their other ends are connected to a mixing pipe 6 to gather the exhaust gases. The runners 2, 3, 4, and 5 have an oval cross-sectional shape.
The exhaust gases generated in engine cylinders are gathered in the mixing pipe 6 through the runners 2, 3, 4, and 5, then directed to a catalytic converter installed in an exhaust pipe (not shown), after which the exhaust gases are discharged from the engine cylinder. The catalytic converter reduces the pollutants in the exhaust gases by typically passing the exhaust gases through a mass of small beads which act as catalysts in converting the pollutants to less harmful gases.
Generally, the catalysts in the catalytic converter chemically react well with exhaust emissions at a light-off temperature (LOT) of about 350.degree. C. At this temperature level, the conversion rate of the catalysts reaches 98%.about.99%. However, the conversion rate decrease rapidly below the light-off temperature.
For the conventional exhaust manifold, since the first and fourth runners 2 and 5 are connected to the mixing pipe 6 at an angle of approximately 130.degree..about.135.degree. (A), the exhaust gases lose heat while passing through this area by the friction of gases resulting from contacting the wall of the mixing pipe 6 during vehicle start-up period. Accordingly, the amount of time it takes for the catalysts to reach the light-off temperature is delayed.
Furthermore, since the first and fourth runners 2 and 5 direct towards the opposite sides of the mixing pipe 6, the exhaust gases from the first and fourth runners 2 and 5 collide into each other then rebound to collide into the inner wall of the mixing pipe 6. This further causes heat loss and acts to delay the time for the catalysts to reach the light-off temperature. This delay in catalyst activation increases toxic exhaust emissions during the period in which the vehicle is warming up.
In addition, after the exhaust gases collide against the wall of the mixing pipe 6, the exhaust gases follow along the wall and are directed to the catalytic converter. As a result, the exhaust gases do not pass evenly through the catalytic converter, but instead are directed only through a portion of the catalytic converter such that the entire active area of the catalytic converter is not utilized. This greatly reduces the conversion efficiency of the catalytic converter.